bna: remove global bnad_list_mutex

Remove global bnad_list_mutex as it is not used anymore. This makes
bnad_add_to_list() and bnad_remove_from_list() empty so remove them too.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>